Here is another vote for ocean currents!
I am helping a friend get set up for the singlehanded Bermuda race in June. The race rules do not permit any paid subscription weather data (freely available data is ok), but I have not found any free source for ocean current GRIBs.
I like the GRIB.US client software a lot! You have done a good job of keeping the interface clean and intuitive (I'm a software developer). I have one other suggestion: I would like to be able to place marks/icons on the map at specific lat/long coordinates. These could represent the vessel or other vessels, or just reference points for plotting other weather information.
Keep up the good work!